This invention relates to a polymer solid electrolyte composition. More precisely, this invention relates to a polymer solid electrolyte composition containing a cold-melting salt and an aluminium-based conductive carrier and thus can exhibit a high ionic conductivity with good film-forming properties, mechanical strength and flexibility.
Use of solid electrolytes to constitute totally solid cells contributes to improving the reliability of the cell without leakage of the content in the cell. Since the cell can be made thin and a plurality of cells may be built up, attention has been directed to solid electrolytes for use as a material in the fields of cells and other electrochemical devices.
The characteristic properties as a solid electrolyte generally include (a) a high ionic conductivity without involving any electron conductivity, (b) good film-forming properties by which a thin film can be formed, and (c) good flexibility.
Broadly, solid electrolytes can be divided into two groups including an inorganic group and an organic group. The inorganic solid electrolytes have a relatively high ionic conductivity but are poor in the mechanical strength because they are crystalline in nature. This makes it difficult to obtain flexible films. This is very disadvantageous when inorganic solid electrolytes are applied to devices.
In contrast, polymer solid electrolytes made of organic materials are able to form flexible thin films. The thus-formed thin film is imparted with good mechanical properties owing to the flexibility inherent to polymers. The thin film consisting of the polymer solid electrolyte can be appropriately adapted for the volumetric variation caused by the ion-electron exchange reaction between the electrode and the polymer solid electrolyte. For these reasons, the polymer solid electrolytes have been expected as a promising solid electrolyte material for high energy density cells, particularly, thin cells.
Composite materials comprising polyethylene oxide ((--CH.sub.2 CH.sub.2 O--).sub.n, hereinafter referred to as PEO) having a polyether structure and alkali metal salts such as Li salts, Na salts and the like are known as polymer solid electrolytes which exhibit a high alkali metal ionic conductivity. Various types of polymer solid electrolytes including the above-mentioned composite materials have been theoretically studied with respect to their mechanisms of ionic conduction and their molecular structures. Extensive studies have also been made on the application of the polymer solid electrolytes to electrochemical devices such as cells.
The ionic conduction of polymer solid electrolytes is now considered to occur in the following manner: the alkali metal salt in the polymer matrix selectively ionizes the amorphous sites in the polymer matrix and moves by diffusion along the electric field in the matrix thereby achieving the ionic conduction while interacting with the coordinating atoms in the polymer. For instance, it has now been accepted that with composite films made of PEO and alkali metal salts, the alkali metal ions interact with the oxygen atom at the ether bond of the main chain of the polymer which has a high dielectric constant, while the molecule chain of the polymer suffers the segment movement by means of the heat at its amorphous sites, thereby showing the ionic conductivity.
However, the polymer solid electrolytes have a problem in that they are smaller in the ionic conductivity in the vicinity of room temperature than solid electrolytes made of inorganic materials. In addition, the improvements in the ionic conductivity of the polymer solid electrolytes brings about a problem in that their film-forming properties and flexibility are lowered instead.
For instance, with the composite material film consisting of PEO and alkali metal salts wherein the composite material has a molecular weight of about 10,000, good film-forming properties are obtained with an ionic conductivity being as high as 10.sup.-3 to 10.sup.-4 S/cm at temperatures of 100.degree. C. or higher. Since the composite material is crystalline in nature, however, its ionic conductivity abruptly lowers at temperatures not higher than 60.degree. C. and is decreased to a very small value of not higher than approximately 10.sup.-7 S/cm at room temperatures. This disenables the composite material film consisting of PEO and alkali metal salts to be used as a material for ordinary cells which are employed at room temperature.
In order to improve the ionic conductivity of the composite material film, an attempt has been made to suppress its crystallinity by making PEO react with toluene diisocyanate (TDI) in the manner shown by the following formula (5), thereby forming urethane-crosslinked structures at the terminal OH groups of PEO. ##STR1##
In addition, another attempt has been made also to suppress the crystallinity of the composite material film by crosslinking PEO with esters. The formation of such crosslinked structures is effective as the means for improving the mechanical characteristics of the composite material film itself without significantly lowering the ionic conductivity of the amorphous polymer in the film. Even by such means, however, the improvement in the ionic conductivity of the composite material film is not as yet satisfactory.
On the other hand, the ionic conductivity of the composite material film at temperatures in the vicinity of room temperature can be improved by making the molecular weight of PEO, which is the polymer constituting the film, lower than 10,000. In this case, however, the film-forming properties of the composite material are considerably lowered, making it difficult to form a film from the material.
Moreover, for improving the ionic conductivity of the composite material film, the concentration of the alkali metal salts therein may be increased. However, this will cause the glass transition point, Tg, of the composite material film to increase, thus rather resulting in lowering of the ionic conductivity of the film. As will be apparent from the foregoing, it is not possible to increase both the carrier density and the ionic conductivity of the composite material film.
Other types of polymer solid electrolytes are known, which are similar to the above-mentioned composite materials consisting of PEO and alkali metal salts, but containing an acrylic or methacrylic, organic high polymer having a PEO structure at its side chain, as shown by the following formula (6): ##STR2## wherein m and n each are a desired integer. In addition, also known are still other types of polymer solid electrolytes which contain a polyphosphazenic, organic polymer having PEO structures as its side chains and having --P.dbd.N-- as its main chain, as shown by the following formula (7): ##STR3## wherein m and n each are a desired integer; as well as those which contain a siloxanic, organic polymer having a PEO structure at its side chain and having --SiO-- as its main chain, as shown by the following formula (8): ##STR4## wherein m and n each are a desired integer.
These polymer solid electrolytes comprising such organic polymers and alkali metal salts have an ionic conductivity ranging from 10.sup.-5 to 10.sup.-4 S/cm and are thus slightly improved over the composite material consisting of PEO and alkali metal salts. However, this ionic conductivity is not satisfactory in practical applications. In addition, their film-forming properties and flexibility are not satisfactory.
On the other hand, attention has been paid to lithium metal secondary cells, lithium ion secondary cells and nickel hydrogen secondary cells for use as high-capacitance cells. Now, there is a strong demand for developments of materials for secondary cells which are small in size, light in weight and high in capacitance, with the recent popularization of portable appliances. One of newly developing cells of such kinds has an anode made of aluminium metal. Theoretically, cells having an anode made of aluminium metal are assumed to have a possibility capable of realizing a high density capacitance which is as high as four times per volume that of conventional lithium ion secondary cells. The production costs for such aluminium cells may be lowered. In view of these, cells having an anode made of aluminium metal are considered promising ones.
However, no practical electrolytic cells have heretofore been known, in which the cell system is composed of an anode made of aluminium metal and an electrolytic solution. The reasons are considered to be the following (a) and (b):
(a) Thermodynamically, aluminium is much more hardly reduced than hydrogen, so that the electrochemically-reversible reaction of aluminium in an aqueous electrolytic solution cannot be expected. For this reason, a non-aqueous cell must be constructed if aluminium is desired to be used as the anode therein. PA1 (b) Since the surface of aluminium has a highly-insulating, firm and dense, passivated oxide film thereon, the dissolution of aluminium by discharging is extremely difficult so that the discharging characteristics of the cell having aluminium as its anode are lowered. In addition, the precipitation of aluminium by charging is also difficult because of the same reason so that the charging characteristics of the cell are also lowered.
On the other hand, to obtain secondary cells which are small-sized and light-weight while having high capacitance, attention has been paid to new materials for cells such as those mentioned below. For instance, a report has been made, stating that certain types of pyridinium or imidazolium quaternary ammonium salts and aluminium chloride can form molten salts at room temperature (cold-melting salts) at certain constitutive ratios and the salts exhibit a very high ionic conductivity. The reported system has been specifically noted and studied as an electrolytic solution for cells. However, the system still has a problem in that the cells using this can function only at elevated temperatures so that these cannot be put to practical use at room temperature or at temperatures lower than room temperature.